The measurements of flow patterns in the channels of micro devices such as mixers and heat exchangers are usually performed with the well-established mu PIV ( micro Particle Imaging Velocimetry) technique. Yet, the micro particles in this technique introduce particle fouling ( PF), which is observed by the inherent active imaging system. This phenomenon is enhanced during the active measurement, leading to erroneous signals long before the flow is blocked. Investigating the roots of PF in micro devices and selecting appropriate means to mitigate or delay it is essential in order to achieve sufficient experimental running time. The history of development on the fouling effect is discussed, thereby emphasizing the influence of pumps, flow guidance, surface roughness and chemistry, fluid chemistry, supersonic disintegration, and particle aging by agglomeration on PF. The cleaning procedures are also discussed. Furthermore, the capability of mu PIV to detect early deposits and their impact on the flow is demonstrated.